1. Field of the Invention
The present invention relates to an improvement in or relating to mounting of an earth or terrestrial magnetism sensor for use in a bearing detector for a vehicle more particularly, the invention relates to an improved mounting of an earth magnetism sensor for use in a bearing detecting apparatus to be provided in an automobile vehicle, which is designed for visually indicating a current bearing or azimuth of the vehicle while following a desired route of travel by determining a relative relation (for instance, an angular or azimuthal relationship) between a current sensed horizontal component of the earth magnetism as detected from time to time and a current travel direction taken by the vehicle in its travel along the desired route.
2. Description of the Prior Art
It is generally known, as typically shown in FIG. 1, of the accompanying drawings that there is normally found a magnetic deviation or declination, i.e., a magnetic declination ".theta." varying in accordance with a given terrestrial spot between an azimuth or bearing reading as given by a magnetic direction finder which is located on that spot (as indicated in dotted line) and a given direction or orientation as taken by way of a map (as indicated in solid line). For instance, it is known in Japan that there is typically observed an extent of magnetic declination ranging from 6.degree. to 10.degree., which varies from place to place as large as several units of tens of degrees according to the terrestrial location in the world. For this reason, it is inevitable that there would occur a certain extent of angular deviation which corresponds to a given magnetic declination of ".theta." particular to a spot of observation from a desired orientation by way of a map, when detecting a current travel direction taken by an automotive vehicle by using a magnetic direction finder which employs a terrestrial magnetism sensor which is designed to be provided on an automobile.
It is known that there have been developed apparatus for the determination of a current travel direction of an automotive vehicle, and visual indication thereof to the driver, which comprises a bearing detecting unit employing an earth magnetism sensor adapted to detect a current orientation and an extent of azimuthal deviation of the vehicle. Generally, the typical construction of such bearing detecting unit which incorporates an earth magnetism sensor is as shown in FIG. 2 of the accompanying drawings. Such bearing detecting unit comprises an element adapted to detect a magnetism component along the direction of the X-axis (for instance, the terrestrial "east" direction) only in terms of the X-Y coordinate plane and another element adapted to detect a magnetism component extending along the direction of the Y-axis (for instance, the terrestrial "north" direction). The thus-detected X- and Y-axis magnetism components are combined vectorwise, whereby a current due orientation of the earth magnetism can be detected properly in terms of the horizontal X-Y plane.
With the employment of a bearing detecting apparatus which incorporates an earth magnetism sensor, it is generally constructed, as schematically shown in FIG. 3 of the accompanying drawings, such that an electric signal for bearing detection of an automotive vehicle which is outputted from an earth magnetism sensor 1 is processed by a signal processing circuit 2, whereupon an indicating unit 3 is driven so as to provide a due indication of a current bearing of the vehicle.
When the bearing detecting apparatus of such construction is installed on a vehicle, it is preferred that the earth magnetism sensor 1 of such apparatus be placed in such a manner that it may be exempted from a possible influence of magnetization of the vehicle's body under the effect of magnetism from a steel structure such as a railroad of the DC-supply type through which the vehicle passes, such as when passing across a railroad crossing thereof as described hereinbelow. To this end, the sensor 1 is installed with its Y-axis (its X-axis) extending horizontally in coincidence with the running direction of the vehicle so as to visually indicate a current heading direction or azimuth of the vehicle by way of the visual display 3. In such arrangement, however, there would inevitably occur a certain error in indication of the current bearing detection on the display between a then azimuth of the vehicle as indicated by way of the display 3 and a given direction on the map to such an extent of angular deviation as involved from a certain magnetic declination ".theta." which is particular to a point of observation as stated above, if the Y-axis of the earth magnetism sensor 1 is put in coincidence with the current travel direction of the vehicle.
When the vehicle is under such conditions, it is very difficult or even impossible for the vehicle's driver to determine whether the displayed information on the screen is erroneous or his vehicle is actually moving in a different or improper way, thus resulting in substantial ambiguity.
In this respect, the present invention overcomes the inconveniences and difficulties encountered in the conventional bearing detecting apparatus for use in an automotive vehicle, which problems have not heretofore been attended to with any effective countermeasures therefor.